Accelerators are used to accelerate charged atomic-sized particles such as ions to very high velocities such that the particles assume the form of a "beam". Beams of energetic particles of this type have a variety of uses in research, medical, industrial and military applications. For example, ion accelerators are used for ion implantation, neutron and radioactive nuclei generation, and in the study of various nuclear and atomic processes. A common device for accelerating charged particles to high velocities applies high frequency alternating electric fields using four poles (or a quadrupole). With the electric fields alternating at radio frequency levels, this type of particle accelerating device is also known as a radio frequency quadrupole (RFQ) linear accelerator. RFQ accelerators are relatively simple in construction and operation, compact, lightweight and portable. These devices are characterized as being capable of accepting large quantities of ions with low kinetic energies and accelerating them to higher energies. The beam accelerated by the RFQ device is generally highly focused due to the strong quadrupole electric field focusing effect.
One of the major disadvantages of current RFQ accelerators is that they are constructed of very precisely made components which are imprecisely mounted within an outer accelerator shell. Complex adjustment mechanisms are required to align the vane tips of the RFQ accelerator in the precise spatial relationship required for uniform field distributions in the four quadrants of the RFQ accelerator. Precise alignment is achieved only by repeated testing and repositioning of the RFQ vane tips. An example of this type of RFQ accelerator can be found in U.S. Pat. No. 4,949,047 issued Aug. 14, 1990.
In an effort to achieve higher surface electric fields for accelerating the particles to higher energies, RFQ devices comprised of superconducting materials have been proposed. Superconducting structures are characterized by high intrinsic Q and, correspondingly, narrow bandwidth. In addition, for low current applications where beam loading is negligible, superconducting structures are sensitive to frequency variations caused by external noise and microphonics (vibration), and ponderomotive instabilities (RF forces). This problem has been addressed in the prior art by a combination of electronic control and mechanically stable geometries. In addition, manufacturing techniques used for superconducting structures are different than those used for normal conducting structures and have an impact on the electromagnetic and mechanical design. For example, demountable joints and sliding contacts may prove to be impractical in superconducting RFQs, with the amount of adjustment likely to be limited to mechanical deformation. Thus, the superconducting RFQ design must be of high strength to provide the electromagnetic mode purity necessary for the requisite insensitivity to dimensional inaccuracies. Finally, increased cooling is required in superconducting RFQs to remove the heat generated by the increased RF currents, particularly in higher current RFQs which are characterized by a certain amount of beam impingement.
Vane-type RFQ structures with four-fold symmetry typically have dipole electromagnetic resonances very close in frequency to the quadrupole resonance which is used for accelerating and focusing the ion beam. Such close-lying modes give rise to small mechanical distortions or errors in the resonant cavity resulting in large changes in the electromagnetic fields. Thus, the resonant cavity structure must be constructed to very tight mechanical tolerances which is particularly difficult when using the fabrication and processing methods required for superconducting materials.
The present invention addresses the aforementioned problems encountered in the prior art by providing a superconducting RFQ device having a simplified geometry which makes use of eight simple "T" sections, provides large dipole-quadrupole mode splitting with a quadrupole mode lower in frequency than the dipole mode, has four-fold rotation symmetry, and affords a high degree of mechanical stability and tolerances.